Dual fuel injector and engine using same

ABSTRACT

A dual fuel injector may be used to inject both gas and liquid fuel into a cylinder of a compression ignition engine. An injector body defines a first set of nozzle outlets, a second set of nozzle outlets, a first fuel inlet and a second fuel inlet. A dual solenoid actuator includes a first armature, a first coil, a second armature and a second coil that share a common centerline. The dual solenoid actuator has a non-injection configuration at which the first armature is at an un-energized position and the second armature is at an un-energized position. The dual solenoid actuator has a first fuel injection configuration at which the first fuel inlet is fluidly connected to the first set of nozzle outlets, the first armature is at an energized position and the second armature is at the un-energized position.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a Divisional of U.S. patent application Ser.No. 13/105,138 filed May 11, 2011, entitled “DUAL FUEL INJECTOR ANDENGINE USING SAME,” which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present disclosure relates generally to dual fuel engines, and moreparticularly to a dual fuel injector with a coaxial dual solenoidactuator for fueling an engine with gaseous and liquid fuels.

BACKGROUND

Gaseous fuel engines are known for their ability to burn clean relativeto their compression ignition engine counterparts. However, gaseousfuels are well known for the difficulty in attaining successfulignition. Some gaseous fuel engines utilize a spark plug, whereas otherengines are known for utilizing a small amount of distillate diesel fuelthat is compression ignited to in turn ignite a larger charge of gaseousfuel. Practical spatial limitations in and around an engine often makeit difficult to find space for all of the plumbing and hardwareassociated with supplying two different fuels to each combustionchamber. U.S. Pat. No. 7,373,931 teaches a dual fuel engine thatutilizes a small quantity and compression ignited distillate diesel fuelto ignite a larger charge of gaseous fuel. This reference teaches theuse of a fuel injector with nested needle valve members to facilitateinjection of both the gaseous and liquid fuels from the same injectorinto each engine cylinder. However, the structure of the injector canlead to cross leakage between fuels, leakage of fuel into the enginecylinder and stacked tolerances that may lead to substantial performancevariations when the fuel injectors are mass produced. In addition, theinjector structure inherently requires different injection patternsdepending upon whether the fuels are being injected individually or atthe same time.

Apart from the potential problems noted above, there may also be issueswith regard to packaging a dual fuel injector with two electricalactuators and control valves in the limited space available in acylinder head mounting of an engine. The '931 patent shows side-by-sideelectronically controlled valves for separately controlling gaseous fueland liquid fuel injection events. In some instances, there may not beenough available space to provide for a side-by-side mounting as shownin this reference.

The present disclosure is directed toward one or more of the problemsset forth above.

SUMMARY OF THE DISCLOSURE

In one aspect, a fuel injector includes an injector body that defines afirst set of nozzle outlets, a second set of nozzle outlets, a firstfuel inlet and a second fuel inlet. A dual solenoid actuator includes afirst armature, a first coil, a second armature and a second coil thatshare a common centerline. The dual solenoid actuator has anon-injection configuration at which the first armature is at anun-energized position and the second armature is at an un-energizedposition. The dual solenoid actuator has a first fuel injectionconfiguration at which the first fuel inlet is fluidly connected to thefirst set of nozzle outlets, the first armature is at an energizedposition and the second armature is at the un-energized position. Thedual solenoid actuator has a second fuel injection configuration atwhich the second fuel inlet is fluidly connected to the second set ofnozzle outlets, the first armature is at the un-energized position andthe second armature is at an energized position.

In another aspect, an engine includes an engine housing that defines aplurality of cylinders. A dual fuel system includes a plurality of fuelinjectors, each including an injector body defining a first set ofnozzle outlets and a second set of nozzle outlets positioned for directinjection into one of the plurality of cylinders. The duel fuel systemincludes a first fuel common rail fluidly connected to a first fuelinlet of each of the plurality of fuel injectors, and a second fuelcommon rail fluidly connected to a second fuel inlet of each of theplurality of fuel injectors. Each of the plurality of fuel injectorsincludes a dual solenoid actuator that includes a first armature, afirst coil, a second armature and a second coil that share a commoncenterline.

A method of operating an engine includes a step of injecting a firstfuel into an engine cylinder through a first set of nozzle outlets ofone of a plurality of fuel injectors. A second fuel is injected into theengine cylinder through a second set of nozzle outlets of the one of theplurality of fuel injectors. Each of the injecting steps includes movingone of a first armature and a second armature toward an other of thefirst armature and the second armature along a common centerline. Thefirst fuel is ignited by compression igniting the second fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an engine and dual fuel common rail systemaccording to the present disclosure.

FIG. 2 is a side sectioned view of a portion of the dual fuel system ofFIG. 1;

FIG. 3 is a sectioned side view of a top portion of one of the dual fuelinjectors from FIG. 1;

FIG. 4 is a sectioned side view of a bottom portion of a fuel injectoraccording to one aspect of the present disclosure;

FIG. 5 is a sectioned side bottom portion view of a fuel injectoraccording to another aspect of the present disclosure; and

FIG. 6 is a series of graphs showing control valve positions, gaseousand liquid fuel rail pressures and injection rates versus time for thedual fuel system of FIG. 1 when operating in a dual fueling mode and alimp home mode.

DETAILED DESCRIPTION

Referring to FIG. 1, an engine 5 according to the present disclosureutilizes a dual fuel common rail system 10. Engine 5 includes an enginehousing 6 that defines a plurality of cylinders 7, only one of which isshown. The dual fuel system 10 includes a plurality of dual fuelinjectors 12 (only one shown) that each include an injector body 70 witha tip component 71 positioned for direct injection of gaseous fueland/or liquid fuel into one of the engine cylinders 7. The dual fuelsystem 10 includes a plurality of outer tubes 50 and inner tubes 40 thateach extend into engine housing 6 between a quill 30 and one of the fuelinjectors 12. Each of the inner tubes 50 is compressed between a conicalseat on an associated quill 30 and a conical seat on one of the fuelinjectors 12. Thus, each engine cylinder 7 has one associated fuelinjector 12, one outer tube 40, one inner tube 50 and one quill 30. Thedual fuel system 10 includes a gaseous fuel common rail 16 that isfluidly connected to each of the fuel injectors 12 through one of thequills 30 and an outer passage 49 defined between an inner tube 50 andan outer tube 40. A liquid fuel common rail 14 is fluidly connected toeach of the fuel injectors 12 through one of the quills 30 and an innerpassage 51 defined by the inner tube 50.

An electronic controller 15 is in control communication with each of thefuel injectors 12 to selectively control the timing and quantity of bothgaseous and liquid fuel injection events. Electronic controller 15 isalso in control communication with a gas pressure control device 20 thatis operably coupled to control the pressure in gaseous fuel common rail16, and also in control communication with a liquid pressure controldevice 22 operably coupled to control the pressure in liquid fuel commonrail 14. Although individual gases, such as methane, propane and thelike are within the scope of the present disclosure, natural gascontaining a mixture of gas species is particularly applicable to thepresent disclosure. In addition, the liquid fuel is chosen for theability for compression ignition at the compression ratio of engine 5.For instance, the liquid fuel may be distillate diesel fuel or someother liquid fuel that is suitable for compression ignition to in turnignite a charge of gaseous fuel in one of the engine cylinders 7.

In the illustrated embodiment, natural gas is maintained in a liquidstate in a cryogenic liquefied natural gas tank 21. A variabledisplacement cryogenic pump is controlled by electronic controller 15 topump liquefied natural gas through filters and a heat exchanger forexpansion into a gas that is maintained in an accumulator. The gaspressure control device 20 according to the present disclosure mayinclude an electronically controlled valve that supplies a controlledquantity of gaseous fuel from the supply side (accumulator) to thegaseous fuel common rail 16. This described supply strategy for naturalgas is particularly suitable when engine 5 is mounted on a movingmachine, such as a mining truck or the like. On the otherhand, if engine5 were stationary, a gas pressure control device may be connected to asource of available natural gas and then compressed and fed to gaseousfuel common rail 16 in a manner that is controlled by electroniccontroller 15 to maintain a desired pressure in the rail 16.

The liquid fuel supply to liquid fuel common rail 14 begins at a tank23. In the illustrated embodiment, the liquid fuel pressure controldevice 22 includes a high pressure common rail fuel pump of a type wellknown in the art whose output can be controlled by electronic controller15 to maintain some desired pressure in liquid common rail 14. Anotheralternative might include fixed displacement pump and a rail pressurecontrol valve that returns a quantity of the fuel back to tank 23 inorder to control pressure in liquid fuel common rail 14. Any of thesealternative strategies fall within the contemplated scope of the presentdisclosure.

In the event that engine 5 is utilized in a moving machine, the presentdisclosure contemplates liquefied natural gas tank 21 having a largercapacity (maybe 65% greater volume) than the distillate diesel fuel tank23 in order to account for the expected ratios of consumption from bothtanks when operating in a standard dual fueling configuration in whichmaybe over 90% of the fuel delivery to engine 5 is in the form ofnatural gas and less than 10% in the form of distillate diesel fuel, bymass. This difference in sizing of tanks 21 and 23 also accounts for thedensities of the respective liquids as well as the different heatingvalues of the two fuels, as well as accounting for the fact that thenatural gas is stored as a liquid but injected as a gas, whereas thedistillate diesel fuel is stored and injected as a liquid into engine 5.When operating in a dual fueling mode corresponding to standardoperation, electronic controller 15 is configured to maintain thegaseous fuel common rail at a medium low pressure and the liquid fuelcommon rail 14 at a medium high pressure. If engine 5 is operating in alimp home fueling mode, the electronic controller 15 may be configuredto maintain the gaseous fuel common rail 16 at a low pressure and theliquid common rail 14 at a high pressure. For the sake of clarity, theidentified high pressure is greater than the medium high pressure, whichis greater than the medium low pressure, which is greater than the lowpressure.

Referring to FIG. 2, the dual fuel common rail system 10 includes acoaxial quill assembly 118 fluidly connecting each fuel injector 12 withliquid and gas common rails 14, 16, respectively. Although the conceptsof the present disclosure could apply to a variety of fuels fordifferent types of engines, the illustrated embodiment is particularlysuited for a gaseous fuel engine that utilizes distillate diesel fuelfor compression ignition. In other words, an engine associated with dualfuel common rail system 10 might primarily burn liquefied natural gassupplied from second common rail 16, and ignite that charge in theengine combustion space by compression igniting a smaller charge ofdistillate diesel fuel from common rail 14 during a combustion event.

Coaxial quill assembly 118 includes a quill 30 at least partiallypositioned in a block 120. The quill includes a first fuel passage 32extending between a first fuel inlet 33, which is fluidly connected tofirst common rail 14, and a first fuel outlet 34. Quill 30 also definesa second fuel passage 35 extending between a second fuel inlet 36, whichis fluidly connected to second common rail 16, and a second fuel outlet37. Quill 30 is fluidly connected to rails 14 and 16 using knownhardware (e.g., fittings) and techniques. Fuel from first common rail 14is moved through an engine housing 6 (engine head) via an inner passage51 through inner tube 50, while fuel from second common rail 16 is movedto fuel injector 12 in an outer passage 49 defined between inner tube 50and an outer tube 40. Inner tube 50 may be of a familiar construction tothose skilled in the art, in that it includes rounded or conical endsthat are compressed between a conical seat 38 of quill 30 and an innerconical seat 55 of fuel injector 12. Thus, the fluid passage 51 withininner tube 50 extends between first fuel outlet 34 of quill 30 and aninner fuel inlet 57 of fuel injector 12. Outer tube 40, which may haveno contact with inner tube 50, has an inner diameter larger than anouter diameter of inner tube 50 in order to define an elongate outerpassage 49 that opens on one end to second fuel outlet 37 of quill 30and at its other end to an outer fuel inlet 48 of fuel injector 12.Outer tube 40 includes a rounded or conical end that is compressed intosealing contact with outer conical seat 46 of fuel injector 12. Theouter fuel inlet 48 opens between the inner diameter of tube 40 and theouter surface of inner tube 50. Thus, fuel injector 12 defines an outerconical seat 46 that concentrically surrounds an inner conical seat 55.In addition, the fuel injector 12 includes an inner fuel inlet 57surrounded by the inner conical seat 55, and an outer fuel inlet 48positioned between the inner conical seat 57 and the outer conical seat46.

Outer tube 40 is compressed between quill 30 and the fuel injector 12.In particular, outer tube 40 includes a rounded or conical end insealing contact with outer conical seat 46 and an opposite end receivedin a bore defined by quill 30. One end 41 of outer tube 40 is sealed viaan O-ring 80 that is positioned in a space 45 between outer tube 40 andquill 30. O-ring 80 is maintained in place against the pressure fromsecond common rail 16 by a back up ring 86 held in place by a cap 87threaded to quill 30. Outer tube 40 is compressed onto outer seat 46 offuel injector 12 by an axial force applied to a load shoulder 42 by acompression load adjuster 60 that includes a contact surface 64 incontact with load shoulder 42. Compression load adjuster 60 includesouter threads 65 that mate with a set of inner threads defined by base121 of block 120, and includes a tool engagement surface 62 located inhollow interior 124 of block 120 to facilitate adjusting a compressionload on outer tube 40. Thus, leakage of the second fuel from common rail16 to atmosphere is inhibited by setting a compression load on the outertube 40 with compression load adjuster 60 above a predeterminedthreshold to facilitate a seal at outer conical seat 46, and by sealingthe other end with o-ring 80.

Sealing at opposite ends of inner tube 50 is facilitated by a separateload adjuster 66 that includes threads 68 mated to internal threadsdefined by base 121 of block 120. Load adjuster 66 includes a toolengagement surface 67 located outside of block 20 that facilitatesmovement of compression load adjuster 66 along a common centerline 54.In other words, compression load adjuster 66 pushes along commoncenterline 54 against quill 30 to compress inner tube 50 between conicalseat 38 of quill 30 and conical seat 55 of fuel injector 12. Because oneend 41 of outer tube 40 can slide within quill 30, the respectivecompression loads on inner tube 50 and outer tube 40 can be adjustedindependently to better ensure proper sealing at all of the conicalseats 38, 55 and 46. Thus, leakage of the first fuel originating fromcommon rail 14 into the second fuel is inhibited by setting acompression load on the inner tube 50 above a predetermined thresholdwith compression load adjuster 66. In addition, leakage of the secondfuel from common rail 16 into the first fuel from common rail 14 mayinclude setting the pressure in common rail 14 higher than the pressurein common rail 16. Outer tube 40, inner tube 50, compression loadadjuster 60, compression load adjuster 66, conical seat 38, innerconical seat 55 and outer conical seat 46 all share a common centerline54. Other sealing strategies for one or both of inner tube 50 and outertube 40 apart from that described in relation to the drawings also fallwithin the contemplated scope of the present disclosure.

As shown, quill 30 may be at least partially positioned within block120, which includes a base 121 and a cover 122 that may be attached tobase 121 by a plurality of fasteners 126. Base 121 may include a flangethat facilitates attachment of block 120 to an engine head (housing 6)via bolts (not shown). As shown in the Figures, the first fuel inlet 33and the second fuel inlet 36 of quill 30 may be located outside of block120. A shim 127 may be included to adjust the distance between conicalseat 38 and conical seat 57 to compensate for geometrical tolerances inthe fuel system and engine components. Any of the second fuel thatmanages to leak past O-ring 80 into hollow interior 124 of block 120,may be vented to atmosphere via vent opening 123. Thus, vent opening 123might be eliminated in a case where the fuel in common rail 16 is notgaseous at atmospheric pressure. Except for vent opening 123, hollowinterior 24 may be substantially closed via an O-ring 81 that is incontact with quill 30 and block 120 and surrounds first fuel passage 32.In addition, a second O-ring 82 may be in contact with quill 30 andblock 120 and surround the second fuel passage 35. Thus, vent opening123 extends between hollow interior 125 and an outer surface 125 ofblock 120, which is exposed to atmosphere.

Coaxial quill assembly 118 may also include a flange 83, collar 85 andbolts 84 to facilitate a sealed fluid connection between quill 30 andcommon rail 14. Although co-axial quill assembly 118 is illustrated asincluding a separate block 120 and quill 30, those skilled in the artwill appreciate that the functions and structures of those twocomponents could be merged into a single component without departingfrom the present disclosure.

Referring now to FIGS. 3-5, each of the fuel injectors 12 includes twocontrol valves 76 that are individually actuated via a dual solenoidactuator 100 in control communication with electronic controller 15. Inthe illustrated embodiment, the two control valves 76 are each two wayvalves that open and close respective pressure relief passageways 111and 112 to a low pressure drain outlet 77. As shown in FIG. 1, drainoutlet 77 is fluidly connected to tank 23 via a drain return line 24.Thus, those skilled in the art will recognize that all of the controlfunctions for fuel injector 12 are performed using the liquid fuel as ahydraulic medium in a manner well known in the art. FIGS. 4 and 5 showtwo different versions of a bottom portion of fuel injector 12. FIG. 4shows a version in which the fuel injector has concentric sets of gasnozzle outlets 90 a and a liquid set of fuel nozzle outlets 96 a,whereas FIG. 5 shows a configuration in which the gas nozzle outlets 90b are side by side with the liquid fuel nozzle outlets 96 b. In theembodiment of FIG. 5, liquid needle valve member 78 b moves along acenterline 79 b, and gas needle valve member 73 b moves along acenterline 89 b that is parallel to, but offset from, centerline 79 b.Identical features in the two different fuel injector versions areidentified with the same numerals, but the numerals include an “a” inthe case of the dual concentric configuration of FIG. 4, and include adesignation “b” in the case of the side by side version of FIG. 5. Inboth versions, the respective gas needle valve member 73 and liquidneedle valve member 78 seat at different locations on the same tipcomponent 71 of the injector body 70.

As shown in FIG. 3, a dual solenoid actuator 100 may be utilized forcontrolling the two control valves 76 in different configurations toprovide a noninjection configuration, a liquid or diesel fuel injectionconfiguration, a gaseous fuel injection configuration, and even acombined injection configuration. Dual solenoid 100 is shown in itsnoninjection configuration with a first armature 110 in an unenergizedposition and a second armature 103 in an unenergized position. Firstarmature 110 is connected to a pusher 106 by an armature attachment 107to hold valve member 154 in an upward closed position in contact withflat seat 151 under the action of shared spring 115. When valve member154 is in its upward closed position, pressure in pressure controlchamber 92 (and pressure relief passage 111) is high (rail pressure) andacts upon closing hydraulic surface 61 of gas needle valve member 73 tomaintain it in its downward closed position to close gas nozzle outlets90.

Second armature 103 is connected to a pusher 108 by a second armatureattachment 109 to urge valve member 153 into contact with flat valveseat 150 by shared spring 115. When valve member 153 is in its downwardclosed position, pressure in second pressure control chamber 95 (andpressure relief passage 112) is high (rail pressure) and acts on closinghydraulic surface 58 to help urge diesel needle valve member 78 downwardto close liquid nozzle outlets 96. When armatures 110 and 103 are intheir unenergized positions, coils 102 and 104 may be in respectiveunenergized states. It should be noted that dual solenoid actuator 100utilizes a common or shared stator 105 upon which both coils 102 and 104are mounted. Thus, magnetic flux necessary to move armature 110 orarmature 103, or both is carried by shared stator 105. Valve members 153and 154 may be made from ceramics and may be un-attached to theirrespective pushers 108 and 106.

In order to initiate a gas injection event, dual solenoid actuator 100is changed to a first fuel injection configuration by energizing coil104 to pull armature 110 downward toward an energized position until themovement of pusher 106 (and armature 110) is arrested by a stop (notshown). When this occurs, valve member 154 moves (is pushed off of seatby high pressure) to an open position out of contact with the flat seat151 to fluidly connect pressure control chamber 92 and pressure reliefpassage 111 to low pressure drain 77 via hidden passages shownschematically by dotted lines. When this occurs, the pressure acting onclosing hydraulic surface 61 decreases and is overcome by the pressureacting on opening hydraulic surface 69, causing gas needle valve member89 to move upward to open gas nozzle outlets 90 to the gas fuel inlet 48(FIG. 2). When it becomes time to end the gaseous fuel injection event,coil 104 is de-energized. This allows shared spring 115 to push valvemember 154 back upward into contact with flat seat 151 to block pressurerelief passage 111 to increase pressure on closing hydraulic surface 61,causing gas needle valve member 73 to move downward to close the gas setof nozzle outlets 90.

A liquid fuel injection event may be initiated by energizing coil 102 tomove armature 103 from its unenergized position to its energizedposition closer to coil 102. When this occurs, pusher 108 is movedupward to permit valve member 153 to move to an open position out ofcontact with flat seat 150 due to pressure in relief passage 112. Whenthis occurs, pressure control chamber 95 and pressure relief passage 112become fluidly connected to low pressure drain 77 (via hidden passagesshown schematically by dotted lined) causing the pressure on closinghydraulic surface 58 to drop. When this occurs, the pressure acting onopening hydraulic surface 59 causes diesel needle valve member 78 tomove upward to open the liquid set of nozzle outlets 96 to the liquidfuel inlet 57 (FIG. 2). When it comes time to end a liquid fuelinjection event, coil 102 may be de-energized. Shared spring 115 thenacts on pusher 108 to move armature 103 back upward toward theunenergized position and move valve member 153 back to its closedposition in contact with flat valve seat 150 to close the fluidconnection between pressure control chamber 95 and low pressure drain77. When this occurs, pressure on closing hydraulic surface 58 againrises causing diesel needle valve member 78 to move downward to closethe liquid set of nozzle outlets 96.

Because dual solenoid actuator 100 can cause valve member 154 and 153 tomove to their open positions independently, the dual solenoid actuator100 also can facilitate a combined injection configuration in which bothcoils 102 and 104 are energized simultaneously. Armature 110, coil 102,armature 103, coil 104 pusher 106, valve member 154, pusher 108 andvalve member 153 may share a common centerline 88. It should be notedthat whenever an injection occurs, one of the armatures 110 and 103moves toward the other of the armature 110 and 103 along commoncenterline 88.

Referring now to FIG. 6, during a gas injection event, one of the twocontrol valves 76 is actuated to fluidly connect a pressure controlchamber 92 to drain outlet 77. In other words, valve member 154 movesinto and out of contact with valve seat 151 responsive to movement ofarmature 110 between an unenergized position and an energized position,respectively. When this is done, pressure in control chamber 92 dropsallowing a gas needle 73 to lift toward an open position against theaction of a biasing spring to fluidly connect a gas nozzle chamber 91 togas nozzle outlets 90. When fuel injector 12 is in a gas injectionconfiguration, the liquid fuel common rail 14 is fluidly connected todrain outlet 77 since pressure control chamber 92 is always fluidlyconnected to a liquid nozzle supply passage 98 through a small orifice.Liquid nozzle supply passage 98 is always fluidly connected to innerfuel inlet 57 (FIG. 2). When the two control valves 76 are in a liquidinjection configuration, the other of the two valves is actuated tofluidly connect the liquid common rail 14 to the drain outlet 77 througha second pressure control chamber 95, which is also always fluidlyconnected to high pressure in liquid nozzle supply passage 98. In otherwords, control valve member 153 moves into and out of contact with valveseat 150 responsive to movement of armature 103 between an unenergizedposition and an energized position, respectively. The two control valves76 also have a combined injection configuration at which both of the twocontrol valves 76 are moved to an open position so that the liquid fuelcommon rail 14 is fluidly connected to the drain outlet 77 through thefirst pressure control chamber 92 and in parallel through the secondcontrol pressure chamber 95. Finally, the two control valves 76 have anon-injection configuration at which the liquid fuel common rail 14 isblocked from the drain outlet 77 by having both of the two controlvalves 76 in a closed position.

In both versions of fuel injector 12 in FIGS. 4 and 5, a gas needlevalve member 73 is positioned completely inside of injector body 70 witha guide surface 75 extending in a guide component 72 of injector body 70between the first pressure control chamber 92 and the gas nozzle chamber91. The gas nozzle chamber 91 is always fluidly connected to the gaseousfuel common rail 16, and is therefore at about the same pressure as thegaseous fuel common rail 16. A segment 74 of gas needle 73 and the guidecomponent 72 define a portion of an annular volume 94 that is alwaysfluidly connected to liquid common rail 14 via a branch passage that isfluidly connected to liquid nozzle supply passage 98. This structure mayhelp to maintain lubricity and hydraulic locking in the guide clearance93.

INDUSTRIAL APPLICABILITY

The dual fuel common rail system 10 of the present disclosure findsgeneral applicability to any engine that utilizes two fuels in thecombustion space of an associated engine. These two fuels may be thesame fuel at two different pressures, or may, as in the illustratedembodiment be different fuels. Although the present disclosure couldapply to spark ignited engines utilizing appropriate fuels, the presentdisclosure finds particular applicability in gaseous fuel engines thatutilize a relatively large charge of natural gas that is ignited viacompression ignition of a small charge of distillate diesel fueloriginating from common rail 14. The coaxial quill assembly 118 of thepresent disclosure can facilitate movement of both fuels to a fuelinjector 12 mounted in the head 6 of an engine 5 via a single borethrough the engine head associated with each fuel injector 12 of theengine 5. This strategy conserves valuable space in and around theengine.

By utilizing a block 120 that is bolted to the outer surface of theengine head, separate load adjusters 60 and 66 can be utilized toindependently load the inner tube 50 and outer tube 40 onto the conicalseats 57 and 46, respectively of fuel injector 12 to inhibit fuelleakage between the fuels and to inhibit fuel leakage to atmosphereoutside of fuel injector 12, while accounting for slight dimensionaldifferences associated with each fuel injector fluid connection.

When in operation, the first fuel (distillate diesel) at a firstpressure moves from first common rail 14 through the first fuel passage32, through inner tube 50 and into fuel injector 12. The second fuel(natural gas) at a second pressure is moved from the second common rail16 through the second fuel passage 35, through the outer passage 49between outer tube 40 and inner tube 50 and into fuel injector 12.Leakage of the second fuel to the first fuel may be inhibited by settingthe pressure in common rail 14 to a medium high pressure (maybe about 40MPa) that is higher than the pressure in common rail 16, which may bemaintained to a medium low pressure (maybe about 35 MPa). Inhibitingleakage of the liquid fuel into the gaseous fuel includes setting acompression load on the inner tube 50 above a first predeterminedthreshold with the compression load adjuster 66 to create appropriatesealing forces on both ends of tube 50. Leakage of the second fuel toatmosphere may be inhibited by setting a compression load on the outertube 40 above a second predetermined threshold with the second loadadjuster 60 to create a seal between outer tube 40 and fuel injector 12.Leakage of gaseous fuel to atmosphere is inhibited by including at leastone o-ring, such as o-ring 80 in contact with outer tube 40.Nevertheless, those skilled in the art will appreciate that otherconcentric tube supply arrangements could be utilized without departingfrom the present disclosure. However, in the illustrated embodiment,leakage and variations in geometrical tolerances in the variouscomponents of engine 5 and fuel system 10 can be accommodated byutilizing first and second compression load adjusters 60 and 66 torespectively adjust the compression loads in the outer tube 40 and theinner tube 50 individually.

The fuel system 10 according to the present disclosure also includesseveral subtle functions providing advantages over known dual fuelsystems. Among these are independent injection control via separatevalves and separate electrical actuators for each of the gas and liquidsystems. Thus, the fuel injector 12 can be controlled to inject gaseousfuel only, liquid fuel only, both gaseous and liquid fuelsimultaneously, and of course have non-injection mode when no injectionoccurs. In addition, the dual solenoid actuator 100 conserves spacewithout sacrificing performance capabilities. Although the migration ofgaseous fuel into the liquid fuel is generally inhibited by maintainingthe liquid fuel common rail 14 at a higher pressure than the gaseousfuel common rail 16, other subtle but important features assist inpreventing such leakage. Cross leakage issues are also inhibited bylocating the liquid fuel supply in the inner tube 50, and locating thegaseous fuel supply to injectors 12 in the outer passage 49 betweeninner tube 50 and outer tube 40. By locating these passagewaysconcentrically, each fuel injector 12 can be supplied with both fuelsvia one passageway through the engine housing 6 (head) rather than twopassageways. Lubricity of the moving components within the fuel injector12 may be maintained by exposure to liquid diesel fuel. For instance,the guide clearance 93 associated with gas needle 73 is maintained withliquid diesel fuel to maintain lubricity, even though one end of the gasneedle 73 is always exposed to gaseous fuel in gas nozzle chamber 91.

By utilizing the concentric supply strategy, the fuel system 10 of thepresent disclosure presents a potential opportunity for retrofittingexisting engines with minimized engine cylinder head modifications. Thestructure of the several versions of fuel injectors 12 also inhibits theleakage of gaseous fuel into the engine cylinder by locating both thegaseous fuel nozzle outlets 90 and the liquid fuel nozzle outlets 96 ina single tip component 71, rather than via some nested needle strategyof a type known in the art. Thus, the fuel injector 12 of the presentdisclosure avoids stacked tolerances and other uncertainties by makingeach of the gas and liquid needle structures independent in theirmovement, seating and biasing features. This strategy may better enablemass production of fuel injectors that perform consistently with thesame control signals. Finally the engine 5 of the present disclosurecontemplates both a normal dual fueling mode and a limp home mode inwhich only liquid fuel is injected. For instance, if a malfunctionoccurs in the gaseous fuel system or if the gaseous fuel supply isexhausted, the electronic controller 15 may cause or allow the engine toswitch from a dual fueling mode to the limp home mode.

As best shown in FIG. 6, the dual fueling mode is characterized by alarge gas injection quantity 138 and a small quantity injection 135 ofliquid fuel. On the otherhand, the limp home mode may be characterizedby no gas injection but a large quantity 136 liquid fuel injection. Inaddition, the normal dual fueling mode is characterized by the gas andliquid common rails 16 and 14 being maintained at medium low and mediumhigh pressures, respectively. On the otherhand, the limp home mode maybe characterized by the gaseous fuel common rail being allowed to decayto, or be maintained at, a low pressure, while pressure in the liquidcommon rail 14 is increased to a high pressure 133 (maybe greater than100 MPa). When operating in the dual fueling mode, a relatively smallinjection of liquid distillate diesel fuel is compression ignited to inturn ignite a relatively large charge of gaseous fuel, which may atleast partially have been previously injected into the engine cylinder.On the otherhand, during a limp home mode, engine 5 functions as asomewhat conventional diesel engine in which a relatively large quantityof liquid fuel is injected at or around top dead center of thecompression stroke to instantaneously ignite upon injection in a knownmanner.

The present description is for illustrative purposes only, and shouldnot be construed to narrow the breadth of the present disclosure in anyway. Thus, those skilled in the art will appreciate that variousmodifications might be made to the presently disclosed embodimentswithout departing from the full and fair scope and spirit of the presentdisclosure. Other aspects, features and advantages will be apparent uponan examination of the attached drawings and appended claims.

What is claimed is:
 1. A method of operating an engine comprising thesteps of: injecting a first fuel into an engine cylinder through a firstset of nozzle outlets of one of a plurality of fuel injectors; injectinga second fuel into the engine cylinder through a second set of nozzleoutlets of the one of the plurality of fuel injectors; each of theinjecting steps includes moving one of a first armature and a secondarmature toward an other of the first armature and the second armaturealong a common centerline; and igniting the first fuel by compressionigniting the second fuel.
 2. The method of claim 1, wherein the firstfuel is natural gas and the second fuel is diesel fuel.
 3. The method ofclaim 1, wherein the step of injecting the first fuel includes moving afirst control valve member along the common centerline; and the step ofinjecting the second fuel includes moving a second control valve memberalong the common centerline.
 4. The method of claim 1, wherein theinjecting steps are performed simultaneously.
 5. The method of claim 4including a step of biasing a first control valve member of each of theplurality of fuel injectors toward a closed position with a sharedspring; and biasing a second control valve member of each of theplurality of fuel injectors toward a closed position with the sharedspring.
 6. The method of claim 5, wherein the step of injecting thefirst fuel includes moving the first control valve member along thecommon centerline, and the step of injecting the second fuel includesmoving a second control valve member along the common centerline.
 7. Amethod of operating an engine comprising the steps of: injecting a firstfuel into an engine cylinder through a first set of nozzle outlets ofone of a plurality of fuel injectors; injecting a second fuel into theengine cylinder through a second set of nozzle outlets of the one of theplurality of fuel injectors; each of the injecting steps includes movingone of a first armature and a second armature toward an other of thefirst armature and the second armature along a common centerline sharedwith respective first and second pushers in each of the plurality offuel injectors; controlling, using the respective first and secondpushers, a first and second control valve member corresponding to firstand second needle valve members, the first and second needle valvemembers being further controlled by a shared spring that opens or closesa first pressure relief passage and a second pressure relief passage viathe first and second pushers; and igniting the first fuel by compressionigniting the second fuel.
 8. The method of claim 7, wherein the firstfuel is natural gas and the second fuel is diesel fuel.
 9. The method ofclaim 7, wherein said injecting the first fuel includes moving the firstcontrol valve member along the common centerline, and said injecting thesecond fuel includes moving the second control valve member along thecommon centerline.
 10. The method of claim 7, wherein said injecting thefirst fuel is performed simultaneously with said injecting the secondfuel.
 11. The method of claim 10 further comprising: biasing the firstcontrol valve member of each of the plurality of fuel injectors toward aclosed position with the shared spring; and biasing the second controlvalve member of each of the plurality of fuel injectors toward a closedposition with the shared spring.
 12. The method of claim 11, whereinsaid injecting the first fuel includes moving the first control valvemember along the common centerline, and said injecting the second fuelincludes moving a second control valve member along the commoncenterline.
 13. A method of operating an engine comprising the steps of:injecting a first fuel into an engine cylinder through a first set ofnozzle outlets of one of a plurality of fuel injectors; injecting asecond fuel into the engine cylinder through a second set of nozzleoutlets of the one of the plurality of fuel injectors; each of theinjecting steps includes moving one of a first armature and a secondarmature of a dual solenoid actuator toward an other of the firstarmature and the second armature along a common centerline; coupling afirst pusher and a second pusher to the first armature and the secondarmature, respectively, and operatively to a first control valve memberand a second control valve member, respectively, via a first valve seatand a second valve seat, respectively; moving a first needle valvemember and a second needle valve member by moving the first valve seatand the second valve seat, respectively, using a shared spring to openor close a first pressure relief passage and a second pressure reliefpassage via the first pusher and the second pusher, respectively,wherein the opening and closing of the first pressure relief passage andthe second pressure relief passage opens and closes a first hydraulicsurface of the first needle valve member and a second hydraulic surfaceof the second needle valve member; and igniting the first fuel bycompression igniting the second fuel.
 14. The method of claim 13,wherein the first fuel is natural gas and the second fuel is dieselfuel.
 15. The method of claim 13, wherein said injecting the first fuelincludes moving the first control valve member along the commoncenterline, and said injecting the second fuel includes moving thesecond control valve member along the common centerline.
 16. The methodof claim 13, wherein said injecting the first fuel is performedsimultaneously with said injecting the second fuel.
 17. The method ofclaim 16 further comprising: biasing the first control valve member ofeach of the plurality of fuel injectors toward a closed position withthe shared spring; and biasing the second control valve member of eachof the plurality of fuel injectors toward a closed position with theshared spring.
 18. The method of claim 17, wherein said injecting thefirst fuel includes moving the first control valve member along thecommon centerline, and said injecting the second fuel includes moving asecond control valve member along the common centerline.
 19. The methodof claim 13, wherein the dual solenoid actuator has a non-injectionconfiguration at which the first armature is at an un-energized positionand the second armature is at an un-energized position; the dualsolenoid actuator has a first fuel injection configuration at which afirst fuel inlet is fluidly connected to the first set of nozzleoutlets, the first armature is at an energized position and the secondarmature is at the un-energized position; and the dual solenoid actuatorhas a second fuel injection configuration at which a second fuel inletis fluidly connected to the second set of nozzle outlets, the firstarmature is at the un-energized position and the second armature is atan energized position.
 20. The method of claim 13 further comprising:mounting a first coil of the first armature and a second coil of thesecond armature on a shared stator of the dual solenoid actuator.